Urea is widely used as a topical, sub-surface and foliar fertilizer. Sulfuric acid has also been widely used in the agricultural industry and in other industries for numerous purposes. It is known to be highly corrosive both to metals and animal tissue, including human skin. In the agricultural industry, sulfuric acid has been used as a soil adjuvant, a water penetration improving agent, a herbicide for a wide variety of undesired vegetation, and as a selective herbicide on resistant crops such as onions and garlic.
Previous investigators have observed that urea, sulfuric acid and water can be reacted to form solutions containing mono- and diurea sulfates. This reaction is so exothermic, that it is difficult to control reaction temperature in large volume production plants.
D. F. du Toit found that urea formed certain compounds with oxalic, acetic, hydrochloric, nitric and sulfuric acids, and that the resulting compounds were stable in contact with their solutions at 20.degree. C. Verslag Akad. Wetenschappen, 22, 573-4 (abstracted in Chemical Abstracts, 8, 2346, 1914).
L. H. Dalman expanded on du Toit's work by developing the phase relationships between the solid phase and saturated solutions at 10.degree. C. (50.degree. F.) and 25.degree. C. (77.degree. F.) but, as in the case of du Toit, did not develop or disclose methods capable of handling the high heat of reaction involved in large scale industrial processing. "Ternary Systems of Urea and Acids. I. Urea, Nitric Acid and Water. II. Urea, Sulfuric Acid and Water, III. Urea, Oxalic Acid and Water;" JACS 56, 549-53 (1934).
In the article "Adding Plant Nutrient Sulfur to Fertilizer," Sulfur Institute Bulletin No. 10 (1964), the addition of nutrient sulfur to fertilizers was discussed and it was mentioned that urea reacts with sulfuric acid to form two complexes of urea sulfate which are useful fertilizers.
In U.S. Pat. No. 4,116,664, L. R. Jones disclosed what is referred to therein as a tortuous, multistage process of producing combinations of urea and sulfuric acid in which portions of the sulfuric acid are incrementally added to and reacted with the total amount of urea to be reacted in each of several stages until the total amount of sulfuric acid has been reacted with the urea. The resulting product is unstable and requires further processing. Jones preferably adds water later as required to obtain stability and the desired composition. He discloses that the reaction can be carried out at temperatures of 100.degree. to 225.degree. F. and that if the sulfuric acid is added to the total amount of urea at a rate which is too fast, the temperature goes to about 200.degree. to 225.degree. F. and that a gas is emitted that causes changes in product characteristics such as solidification. The patent states that temperatures of 160.degree. to 200.degree. F. are preferred.
In U.S. Pat. No. 4,310,343, W. J. Verdegaal et al. disclosed a process for making a liquid fertilizer by reacting urea and sulfuric acid and using a heat sink of a "heel" of reaction product to dissipate the heat of reaction.
In U.S. Pat. No. 4,315,763, J. H. Stoller et al. disclosed the preparation of liquid fertilizers from a mixture of urea and phosphoric acids optionally containing the reaction product of urea and sulfuric acid. The patentees suggested that leaving a heel in the reaction vessel permitted further manufacture to be conducted in a stirred fluid mass.
In U.S. Pat. Nos. 4,397,675, 4,402,852, 4,404,116, 4,445,925, 4,447,253, 4,512,813, 4,522,644 and 4,589,925, D. C. Young disclosed processes for preparing urea-sulfuric acid compositions and various uses for such compositions. The processes generally utilized measured quantities of reactants selected to produce a product having a desired composition. The patentee controlled the temperature of reaction mainly by employing countercurrent direct air heat exchange, but suggested that heat dissipation and temperature control were facilitated by assuring that a heel or inventory of reaction product was present during the reaction.